Electron gun for reducing shot noise



Dec. 5, 1961 H. G. KOSMAHL ELECTRON GUN FOR REDUCING SHOT NOISE Filed June 2, 1959 lo V a I i i 0.0. 1+V POTENTIALLV I o.c. VELOCITY -DISTANCE Z "DISTANCE Z FIG.

FIG. 2

FIG. 3

INVENTOR, HEINRICH a. KOSMAHL.

M%dM0701 g 3,012,171 ELECTRON GUN FQR REDUCWG SHOT NOISE Heinrich G. Kosrnahl, Wall Township, N.J., assignor to the United States of America as represented by the Secretary of the Army Filed June 2, 1959, Ser. No. 817,696

3 Claims. (Cl. SIS-14) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to improvements in microwave amplifiers and more particularly to low noise electron guns for use in forward and backward wave type travelling wave tubes.

The noise caused by irregularities in the emission of electrons from the cathode of the gun structure used in travelling wave type tubes is known as shot noise. One of the principal objects of the present invention is to provide improved means for smoothing out the deleterious effects of such shot noise. Another and related object of the invention is to smooth out the effect of shot noise before the electron beam inter-- electrons which are accelerated due to space charge forces will get ahead of their normal or equilibrium position, and those which are decelerated will get behind their equilibrium position so that the stream becomes formed into relatively concentrated bunches of electrons which are separated by rarified regions. 7

Owing to the mutual repulsion between electrons, the bunches will spread out and coalesce, and the stream will be of substantially uniform density again at some point along its path. Here the electrons are in their original equilibrium positions with respect to each other, the axial repulsion forces exerted on each electron by those ahead of it being balanced by those behind it. However, the electrons at this point are moving with respect totheir equilibrium positions, and momentum carries them past those positions in the opposite directions from those in which they were originally displaced. Thus, the stream-is bunched again, at some subsequent point, and the cycle repeats itself.

This phenomenon is called plasma oscillation, or space charge oscillation. The frequency f of oscillation of the electrons about their equilibrium positions depends upon the restoring force exerted on an electron per unit of its longitudinal displacement from equilibrium, and is substantially proportional to the square root of the D.C. charge density q. The distance through which the stream moves at the D.C. velocity a during one cycle of such oscillation is and is called the space charge wavelength or plasma wavelength A The quantities f and A represent theoretical values of plasma frequency and plasma wavelength in an electron beam with infinite lateral extensions transverse to the direction of propagation; For example purposes, however, these values are replaced by what are known in the art as reduced quantities which are designated w and mint 3o distance of onehalf the plasma wavelength M, respectively. Accordingly, for purposes of this appliforcing two D.C. potential or velocity minima spaced apart along the beam by one-half the plasma wavelength; For a better understanding of the invention, together with other objects thereof, reference is had to the following description taken in connection with the'accompanying drawings, in which: 7

FIG. 1 is a schematic diagram of an electron discharge device embodying the present invention as it is utilized in a travelling wave tube; and 1 FIGS. 2 and 3 are explanatory curves illustrating th principle of the invention.

Referring now to FIG. 1, there is shown at 10 an electron gun for a travelling wave tube arranged to project a' beam of electrons along a longitudinal axis Z to a collector electrode (not shown). The electron gun includes a cathode 12 and a plurality of aligned apertured electrodes 14, 16, 18, 2t and 22, following cathode 1-2v in that order. The respective apertures in the electrodes 14-22 are aligned so that the longitudinal electron beam passes therethrough. A D.C. voltage source 15 is provided to maintain the apertured electrodes at suitable D.C. potentials with respect to the cathode at zero. The values of the applied D.C. voltages and the spacing of the apertured electrodes along the Z axis of the tube are such that there are produced two D. C. potential minirna spaced a In one practical embodiment of the invention, the spacing between cathode 12 and the first apertured electrode 14 was made approximately 0.1 cm.; the spacing between first apertured electrode 14 and second apertured electrode 16 was made approximately 0.3 cm.; and the respective spacing between apertured electrodes 16, 18, 20, and 22, was made approximately in the order of 0.05 cm. With the potential at cathode 12 as Zero, the D.C. potential applied to apertured electrodes 14 and 16 may be between 1 and 2 volts; the apertured electrodes 18 and 20 are at zero potential; and the apertured electrode 22 is approximately at 20' volts. It is to be understood, of course, that such spacings and potentials are given only as an example and are not to be necessarily limited thereto. Following aperture 22 and disposed adjacent thereto is a tubular member 24 which functions as the conventional focussin-g or accelerating electrode and does not comprise any part of the invention. The entire gun structure is placed in a parallel magnetic field, indicated by the arrows, to maintain parallel electron beam flow. Al-

though not shown, the usual velocity gap and accelerating electrodes may follow tubular member 24.

FIG. 2 indicates the D.C. potential V in the gun as a function of the distance Z along the longitudinal axis. As

shown, the potential first becomes slightly negative with respect to the cathode 12, then increases. up to the 1-2 D.C. voltage applied to apertured electrode 14 from batand again increases up to approximately 20 D.C. volts Patented Dec. 5, 1961 3 which is applied to apertured electrode 22. As explained below, the enforced two potential minima spaced provide for space charge smoothing. FIG. 3 illustrates the variation of the average D.C. velocity along the longitudinal Z axis resulting from the DC potential distribution shown in FIG. 2.

The first potential minimum near the cathode 12 is caused by the space charge of a cloud of electrons which is formed in the vicinity of the cathode. As hereinabove explained, individual electrons are emitted from the cathode at random velocities and at random times. The term D.C. velocity, as used herein means the average velocity of the electrons in the part of the beam under consideration and is designated by n Slow moving electrons are repelled back toward the cathode by earlier emitted slow electrons and fast moving electrons have enough momentum to overcome the repulsion and pass the slower, earlier electrons. Thus, a more or less stationary cloud is formed, with electrons entering it from the cathode, some returning to the cathode, and some going on through the remainder of the tube to the collector anode. The center of the cloud, at the potential minimum, is called a virtual cathode since the electrons which get through it behave as if they emanated from that point. At a distance from the first virtual cathode, a second virtual cathode is enforced by the spaced apertured electrodes 1622 and the potentials applied thereto. While four such apertured electrodes are shown here, it is to be understood that four to six such apertured electrodes may be utilized. The actual members and exact potentials applied to these electrodes to enforce a second virtual cathode at a distance are functions of beam current, noise current, and frequency, and may be determined exactly for each case. The second virtual cathode or potential minimum along the longitudinal axis Z is formed in the same manner as that of the first virtual cathode described above.

To explain this smoothing operation, it is first assumed that the positions and depths of both potential minirna shown in FIG. 2 are not actually stationary but change slightly with the fluctuations of the emission current. Thus their positions and depths may be said to correspond to a moment of time when the emission current is an average one. Suppose now that an excess current is being emitted in a velocity class near the mean D.C. velocity ,ug. After passing the first of the potential minima, this excess A.C. current amplitude, I, will cause the first potential minim-um to move toward the cathode to set up a current wave of the type Where w is the reduced plasma frequency; Z is the longitudinal dimension in the direction of propagation of the beam; and T is the transit time. This current wave will propagate in the drift region between the two potential minima and, when it arrives at the second potential minimum it will have the form and, since The right hand side of Equation 3 may be written as so that Equation 3 now may be written as 1| ](wt.T-wq

Neglecting the time factor, it can be seen from Equation 4 that the current which means that the concentrated bunches of electrons have been converted to anti-bunches, or, conversely, the anti-bunches have been converted to bunches. As a result it can be seen that the response of the second potential minimum will be opposite to, or out of phase, with that of the first potential minimum. This process will be repeated for all other fluctuations in the emission starting at the cathode so that each primary fluctuation will cause two opposite responses at the two potential minimum, the final result providing a smoothening out effect of the shot noise current.

While there has been described What is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A low noise electron gun comprising a cathode for producing an electron beam, a first electrode adjacent said cathode and maintained at a relatively low DC. potential with respect to said cathode whereby there is produced a first DC. potential minima along said beam, and means spaced approximately one-half the plasma wavelength from said first electrode and maintained at the potential of said cathode whereby there is produced a second DC. potential minimum along said beam.

2. The electron gun in accordance with claim 1 wherein said last mentioned means comprises a plurality of parallel spaced electrodes, the spacing between said first electrode and the first of said parallel spaced electrodes being greater than the spacing between said parallel electrodes.

3. In an electron discharge device; means for producing an electron beam, said beam being characterized by undesired shot noise currents; and means for reducing said shot noise comprising a first electrode adjacent said electron producing means and maintained at a relatively low potential with respect to said electron producing means, and a plurality of spaced parallel electrodes in spaced relationship to said first electrode along said beam at a distance substantially one-half the plasma wavelength, said parallel electrodes being maintained at the potential of said electron beam producing means.

References Cited in the file of this patent UNITED STATES PATENTS 2,227,034 Schlesinger Dec. 31, 1940 2,581,243 .Dodds Jan. 1, 1952 2,762,948 Field Sept. 11, 1956 2,800,602 Field et al July 23, 1957 2,800,606 Tien et a1 July 23, 1957 2,869,021 Currie Jan. 13, 1959 

